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Old 11-23-2008, 08:16 PM   #1
makofoto
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Default Solo Stock Exhaust's

For the WRX/DS and STI/AS ... better to run with nothing after the CAT (weight reduction, ? Power/Torque) or CAT Back Exhaust?

How does the dealer installed SPT exhaust compared to whatever else is out there: Cobb, Harmon, etc. $820, not including installation. Most of us would have no problem doing our own install. The SPT is CARB legal.

There is a considerable weight savings in dropping the '08/'09 STI four tipped oem muffler. What happens to one's power with the back pressure reduction?


I was talking to a owner of a MS-R Miata ... who did an actual dyno test. Dropping their muffler lost them 7 hp. Going to an aftermarket sport CAT back lost them only 2 hp. Most HP but also most weight was with the oem muffler. But a couple of HP can deal with 35 lbs nicely. I'm not sure how back pressure effects turbo engines?
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Old 11-23-2008, 08:23 PM   #2
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IMHO, on a Subaru, drop it. The only reason to keep a cat back is for backpressure on an N/A engine. A turbo, not so much.

--kC
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Old 11-23-2008, 08:26 PM   #3
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Don't you mean IMO, not IMHO


I recall now that we DID try dropping Barco's muffler once ... and we thought there might have been a drop of mid-range torque. Perhaps one needs to tune for it?
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Old 11-23-2008, 08:32 PM   #4
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Well Hoppe ran a straight pipe on his DS WRX....loss of Weight and a pick up in HP...

As KC said...On a N/A not so much..

Bill
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Old 11-23-2008, 11:09 PM   #5
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Quote:
Originally Posted by KC View Post
IMHO, on a Subaru, drop it. The only reason to keep a cat back is for backpressure on an N/A engine. A turbo, not so much.

--kC
It's a good thing this isn't the NA forum, or else williaty would have found a way to stab you through a computer.
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Originally Posted by williaty View Post
Seriously, once and for all: BACKPRESSURE IS NOT NECESSARY FOR THE PROPER FUNCTION OF A 4-STROKE NATURALLY ASPIRATED MOTOR AND, IN FACT, HURTS PERFORMANCE.
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Old 11-24-2008, 12:52 AM   #6
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I hate to say it, but have you asked in the 2.5L turbo forums?
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Old 11-24-2008, 06:10 AM   #7
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Give it a try, can't hurt to try, save money, don't buy another pipe. save weight, drop the muffler. My '06 Sti got very happy w/ no muffler. But I have heard it only works w/ '06 '07 stock tune.

One more thing, i've tried every bracket i can find to support the turn down pipe, coat hangers work best!!!!! (you may have seen me under the car as wifey was getting ready to go to grid, broke another bracket, coat hanger going in 5 minutes before she runs)
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Old 11-24-2008, 07:19 AM   #8
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It's a good thing this isn't the NA forum, or else williaty would have found a way to stab you through a computer.
Ok, allow me to rephrase... "Backpressure" is an erroneously used term to describe what is happening, but is commonly used as a catch-all phrase much like using 'Rollerblades' to describe 'in-line skates'. IOW, he's right. It's not backpressure.

Lopping off the exhust on a N/A engine changes the velocity and scavenging of the exhaust flow enough to hurt the performance of the output of the engine/exhaust at low to mid RPMs (where you'll lose the most time on course in a solo-car), but it will allow a little more top end, not enough to really notice tho.

On a turbo car, most of the power comes from the velocity and scavenging effects of the exhust gasses as it flows through a turbo, and spools faster when you can just get rid of the exhaust asap after the turbo, therefore, generating more power, quicker. Sure, there's still scavenging effect going on after the turbo, but it's 'purpose' has been diminished, not like a N/A car.

That better?

--kC
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Old 11-24-2008, 09:11 AM   #9
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Quote:
Originally Posted by KC View Post
IMHO, on a Subaru, drop it. The only reason to keep a cat back is for backpressure on an N/A engine. A turbo, not so much.

--kC
And even then backpressure is a myth. Not to get off the topic...

But we can't have one of our most astute members not on board. Backpressue on an N/A engine, or otherwise is a complete myth. What you need is high exhaust gas velocities to promote gas scavenging.

This high velocity is achieved, normally, through running narrower pipes, especially at the headers. Somewhere along the line, someone equated narrow headers, etc. to backpressure.

In reality, backpressure is bad all around for making power.

http://www.thumpertalk.com/forum/sho...9727&p=6554557
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Old 11-24-2008, 11:49 AM   #10
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Quote:
narrower pipes
But for the post turbo exhaust system, it IS better to go to 3" pipes? Back when I had my '03 WRX I was told by an old time Suby Guru to go with 2.5" pipes, lighter and more velocity?
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Old 11-24-2008, 12:39 PM   #11
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Originally Posted by makofoto View Post
But for the post turbo exhaust system, it IS better to go to 3" pipes? Back when I had my '03 WRX I was told by an old time Suby Guru to go with 2.5" pipes, lighter and more velocity?
If you have to have an exhaust for inspection, driving, and/or sound purposes, most 2.5" pipes will be lighter, and pretty much, unless you're running huge HP and need to move a greater volume as good as a 3" dia. At an event? You can drop them if you don't have to worry about passing sound and there'll be a negligible difference.

--kC
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Old 11-24-2008, 12:39 PM   #12
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Quote:
Originally Posted by makofoto View Post
But for the post turbo exhaust system, it IS better to go to 3" pipes? Back when I had my '03 WRX I was told by an old time Suby Guru to go with 2.5" pipes, lighter and more velocity?
Of course, it's better to run with an open downpipe for that matter.

EG velocity post turbo isn't aiding with scavenging etc.
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Old 11-24-2008, 12:56 PM   #13
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Quote:
Originally Posted by makofoto View Post
But for the post turbo exhaust system, it IS better to go to 3" pipes? Back when I had my '03 WRX I was told by an old time Suby Guru to go with 2.5" pipes, lighter and more velocity?
Haha...That Sounds like Mike Shields....whom never wanted to admit that Technology passed him by...


3in definitely the way to go on Turbo cars


Bill
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Old 11-24-2008, 01:05 PM   #14
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Quote:
Originally Posted by Scooby South View Post
Haha...That Sounds like Mike Shields....whom never wanted to admit that Technology passed him by...


3in definitely the way to go on Turbo cars


Bill
bingo!

http://www.spdusa.com/stock_engine_mods.htm
Quote:
We sell 2.5 inch exhaust pipe for road cars, while many others sell 3 inch. Why? While higher power 300+hp engines can benefit from larger exhaust piping, we have found that fully tuned "pump gas" WRX engine benefits from keeping their standard 2.5 inch exhaust pipe diameter. We are not going to make so much power on the high rpm range that 2.5 inch is a restriction. If we go to 3 inch pipe size we create a very lazy spool-up in the lower rpm range with no gain on the top end on our ordinary super gasoline. This is very practical advice. Remember, SPD Tuning Service could make more money if we recommended you change all your exhaust to 3 inch! It would be louder and more expensive, but would not run any better.
Back in the day that one did get quoted a lot.
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Old 11-24-2008, 01:13 PM   #15
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Quote:
Originally Posted by Scooby South View Post
Haha...That Sounds like Mike Shields....whom never wanted to admit that Technology passed him by...


3in definitely the way to go on Turbo cars


Bill
For stock level auto-x STIs, has there been any dyno results on the differences between 2.5", 3" and just plain removal of the cat-back?

--KC
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Old 11-24-2008, 03:07 PM   #16
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Bill ... you guessed it perfectly!

But I have to say that Mike was very nice ... actually went to his place. He exchanged the Stromung muffler that I bought from him for a ProDrive, when it proved that the Stromung was too loud for my neighbors.
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Old 02-18-2009, 11:12 PM   #17
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I've toyed w/ the idea of a turndown that bolts up- I think an exhaust shop could make one pretty easy and cheap

I do know that the SPT cat back boasts a 10 hp/ 6 ft lb gain (article from SEMA), would be interesting to see what it would sans cat back?

I haven't weighed the oe cat back but am guessing ~ 30 lbs- to bad that wasn't off the other end
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Old 02-19-2009, 12:10 AM   #18
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I weighed my stock catback tonight and it weighs about 39.5 lbs. Didn't weight my SPT, but it didn't feel much lighter to me.
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Old 02-19-2009, 09:20 AM   #19
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40 lbs- hmmmmmmmm might have to look further into the turndown
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Old 02-19-2009, 09:25 AM   #20
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IMHO, as long as you pass sound (if you have sound requirements), it's a no brainer.

--KC
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Old 02-19-2009, 10:51 AM   #21
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In the Los Angeles area you can get a "custom" bolt on turn down for $30, $40 from one of the many low end muffler shops. Pete got one for his '08 STI, I had one made for our Miata. Max Hayter might have paid more for his out the side end pipe.

One DOES have to watch out for the sound meter at Qualcomm in San Diego. Have to be ready with some steel wool and a few cross bolts to hold it in.

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Old 02-19-2009, 11:12 AM   #22
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Quote:
Originally Posted by KC View Post
IMHO, as long as you pass sound (if you have sound requirements), it's a no brainer.

--KC
Keith- I think your right

Mako- that is a fancy one
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Old 02-19-2009, 09:22 PM   #23
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just weighed my Cobb catback (yet uninstalled) and came up w/ 32.5 lbs- so a few lb saving, far from 40 lbs though

what I might do for simplicity sake (and to save time at events) is leave the two Cobb pieces bolted up and just replace the muffler section (two bolts and a couple of hangers) it alone weighed 21 lbs
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Old 02-19-2009, 11:15 PM   #24
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<shrug> I ran a stromung 3" on the DS car. I often wanted to pull the axle-back at events, but never did. Never really noticed a performance benefit, but it was a *lot* lighter, and the sound made me happy
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Old 02-20-2009, 09:06 AM   #25
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I've heard some B Stock Mazda's run sans cat back- sounds like a totally different car
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